fix: replace sub-dict keys on assignment in NDict.__setitem__ (Fixes #397)

[rtmalikian]

Fixes #397

Problem

When assigning a new dict to an existing key in NDict, old sub-keys that were not present in the new dict were retained, causing unexpected merge-like behavior instead of full replacement.

Reproduction:

my_dict = NDict({"f": {"a": 1, "b": 2}})
my_dict["f"] = {"b": 1000}
print(my_dict)
# Actual:   {'f.a': 1, 'f.b': 1000}  ← f.a leaked through
# Expected: {'f.b': 1000}

Root cause: NDict.__setitem__ (line 212) iterates over the new dict's keys and sets them, but never removes the old sub-keys stored under the same prefix.

Solution

Before setting the new dict's sub-keys, delete all existing _stored keys that start with key + ".". This ensures the assignment fully replaces the previous sub-dict content.

Verification

# Exact reproduction from issue
my_dict = NDict({"f": {"a": 1, "b": 2}})
my_dict["f"] = {"b": 1000}
assert dict(my_dict._stored) == {"f.b": 1000}  # PASS

# Edge cases
my_dict2 = NDict({"x": {"a": 1, "b": 2, "c": 3}})
my_dict2["x"] = {}
assert dict(my_dict2._stored) == {}  # PASS — empty dict clears all

my_dict3 = NDict()
my_dict3["z"] = {"a": 1}
assert dict(my_dict3._stored) == {"z.a": 1}  # PASS — new assignment works

my_dict4 = NDict({"f": {"a": {"x": 1}, "b": 2}})
my_dict4["f"] = {"a": {"y": 2}}
assert "f.a.x" not in my_dict4._stored  # PASS — nested replacement
assert my_dict4._stored.get("f.a.y") == 2

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About the Author: Raphael Malikian — Clinical AI Solutions Architect. I specialise in building and fixing AI/ML systems for healthcare, including vector databases, RAG pipelines, and clinical NLP. If you need help with your project or think I can add value to your organisation, feel free to reach out — I'd love to connect.

📧 rtmalikian@gmail.com
🔗 GitHub: https://github.com/rtmalikian
🔗 LinkedIn: http://www.linkedin.com/in/raphael-t-malikian-mbbs-bsc-hons-71075436a

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Disclosure: This code was developed with assistance from mimo-2.5-pro (Xiaomi) via Hermes Agent (Nous Research). All changes were reviewed, tested against the actual codebase, and verified for correctness.